1. Field of the Invention
This invention relates to metal casting, and more particularly to immersible pneumatic weighing dosers for molten metals.
The term "molten metal" will be employed hereinafter to designate both molten pure metals, such as aluminium or magnesium, and alloys, for example, on the base of iron, aluminium or magnesium.
These dosers are adapted for taking molten metal from under the plane of melt in a melting unit and extruding melt doses by a compressed gas into casting molds, for example, of die casting or centrifugal casting machines. It is well known to those skilled in the art that molten metals may be poured into molds not only from the vessels of melting units but also from ladles, mixers and the like.
The term "dose" will be understood hereinafter to mean an amount of molten metal measured in volume or mass units. Thus, the dose measuring in volume units will be understood hereinafter to means "volumetric dosing", and the dose measuring in a mass or weight units will mean "weight dosing". Therefore, immersible pneumatic dosers adapted to accomplish volumetric dosing are called as "volumetric dosers" and immersible pneumatic dosers adapted to provide weight dosing mean "weighing dosers".
2. Brief Description of the Prior Art
Taking of molten metal doses from under the plane of melt in a melting unit and a subsequent extrusion of the doses by compressed gas inert with respect to the melt provide a supply of molten metal practically free of slag and oxides to a casting mold. This is particularly important in manufacturing castings of high quality. Among means for accomplishing this casting method, immersible pneumatic dosers are characterized by low power consumption, small overall dimensions and simple maintanance. However, the problem of dosing by using such dosers has not yet been resolved quite perfectly.
In most cases immersible pneumatic dosers operate on the principle of volumetric dosing. Volumetric dosers are basically similar in construction and comprise a measuring vessel suspended from a bearing element and immersible into a liquid to be dosed and which includes a conduit to let the liquid in, a pipe for discharging a dose, and a pipe for communicating the interior of the vessel with a compressed gas source (see for example, French Pat. Nos. 1204357, 1274944 and Norwegian Pat. No. 130221).
The amount of molten metal contained in immersible pneumatic volumetric dosers is mostly determined either by the capacity of a measuring vessel or by the volume of a compressed gas supplied into the vessel.
Further, the dose size is dependent on some other factors, namely:
the level and respectively the volume of molten metal in the discharge pipe of the measuring vessel at the moment prior to the extrusion of a dose (this volume is a part of the dose); PA1 the partial extrusion of a molten metal through a constantly open opening of the measuring vessel into the melting unit (this effect in the non-valved volumetric dosers invariably takes place at the moment of extrusion of a dose through the discharge pipe); PA1 fluctuations of the melt temperature in a melting unit, in particular a temperature drop as molten metal is taken off; PA1 a change of the inner volume of a measuring vessel as a result of corrosion or of deposit on its walls.
As the molten metal is taken from the melting unit, the melt level lowers therein, and as a result, the molten metal volume in the discharge pipe of the doser decreases and, due to a drop in hydrostatic pressure, the amount of metal extruded through the inlet opening of the measuring vessel back into the melting unit increases. Therefore, each succeeding dose is found to be less than the previous one.
In order to rule out this constantly increasing error in dosing, some attempts have been made for stabilizing the melt level in the melting unit through compensating for the amount of metal taken therefrom either by adding molten metal from an outer source, or (French Pat. No. 1204357) by introducing a compensating body, for example, a compressed gas supplied underneath a bell submerged with its open end into the melt concurrently with the extrusion of doses from the measuring vessel. In any case, the compensation for the melt consumption complicates both the dosing process and the equipment.
The fluctuations of melt temperature affect the pressure of compressed gas used for discharging metal doses and, hence, the accuracy of dosing. The use of an automatic pressure control system (Norwegian Pat. No. 123618) results in complicating the dosing equipment and leads to an increase in its cost.
In general, the problem of improving the dosing accuracy can be more efficiently solved by using the method of weight dosing, wherein the accuracy is not influenced by most of the above factors. The dose size control (within the measuring vessel) presents in this case no special problems.
One weight dosing method is applied in an immersible pneumatic weighing doser (according to USSR Inventor's Certificate No. 431964) comprising, a measuring vessel having a filling opening with a filling pipe passed therethrough into said vessel, the upper end of said pipe being disposed close to the dome of the measuring vessel, and a built-in discharge pipe in the measuring vessel so that its inlet opening is disposed close to the bottom of the vessel. Further, the measuring vessel is provided with a pipe for communicating with a compressed gas source. Said measuring vessel is suspended from a lever secured to a fixed support, said lever being kinematically connected with a weight pick-up. The lever and the pick-up represent the main part of a weighing controller which is to be preliminarily calibrated for the value of buoyancy acting upon the measuring vessel.
The buoyancy value is registered by a measuring device. The term "buoyancy" will be employed hereinafter to designate a resultant value of the buoyancy force acting upon the measuring vessel submerged in the melt and the oppositely directed gravity force of the measuring vessel, which equals to the difference of these forces in absolute values. With a given volume of the measuring vessel, the buoyancy force is a constant, while the gravity force is a variable. When the melt is forced out from the measuring vessel by compressed gas, the gravity force decreases and the buoyancy force increases, respectively. The increment amount of the buoyancy force is equal to the weight of displaced molten metal and does not depend on the molten metal level in the melting unit, fluctuations of molten metal temperature, fluctuations of compressed gas pressure and inner volume changes of the vessel.
However, not the whole amount of molten metal displaced from the measuring vessel gets into the mold. At the starting moment of dosing a part of molten metal passes through the constantly open filling pipe back into the melting unit chamber. It is obvious that a change of hydrostatic pressure in said chamber influences the dosing accuracy.
One of the significant disadvantges of the above-described immersible pneumatic weighing doser is in the depth limits of immersion thereof in the melting unit, since in the lower part of the measuring vessel there is a rather long vertically disposed inlet pipe. Therefore, a considerable portion of molten metal remains in the melting unit. An arbitrary reduction of the filling pipe height is not permissible, as its length is to exceed the height of lift of the molten metal forced out from the measuring vessel through the discharge pipe. Otherwise, the gas may burst through the filling pipe into the chamber resulting in a drop of the working pressure in the measuring vessel and a stoppage of supply of the molten metal into the mold.
Within the limits of the above-described basic diagram of the immersible pneumatic weighing doser for molten metals the displacement of molten metal into the chamber may be prevented and the overall dimensions of the measuring vessel may be reduced only by replacing the filling pipe, which acts as a hydroseal, by a mechanical inlet closing device.
However, as has been practically proved, non-drivable closing devices of a check-valve type are not sufficiently reliable when they are to operate in molten metal, while the closing devices with independent drives do not provide for the required accuracy of dosing. Inadequate accuracy of such devices results from distortions of weighing controller readings caused by the resistance of the closing device elements to the movements of the measuring vessel and the weighing controller lever, when the buoyancy force changes.
Another disadvantage of the described doser is that in the process of discharging the measuring vessel the center of gravity thereof moves in the plane of the lever swing. This movement of the center of gravity changes the arm of the buoyancy force with respect to the rotation axis of the lever. Therefore, the force of the lever action on the stress pick-up also changes, which results in a dosing error.